1. Field of the Invention
The present invention relates to a white organic light emitting device, and more particularly, to a tandem white organic light emitting device having high efficiency and long lifespan by adjusting characteristics of a hole transport layer adjacent to a charge generation layer.
2. Discussion of the Related Art
With the advent of the information age, the field of displays that visually express electric information signals has rapidly developed. Correspondingly, a variety of flat display devices having excellent performance, such as slim design, low weight, and low power consumption, have been developed and rapidly replaced conventional cathode ray tubes (CRTs).
Examples of flat display devices include liquid crystal display device (LCDs), plasma display panel devices (PDPs), field emission display devices (FEDs), and organic light emitting devices (OLEDs).
Among the aforementioned flat display devices, the OLED is considered most competitive because it does not need a separate light source and realizes a compact device design and vivid color reproduction.
Such an OLED essentially requires formation of an organic light emitting layer. In order to form the organic light emitting layer, a deposition method using a shadow mask has been conventionally used.
However, if a shadow mask is used in a large area, the shadow mask sags due to load thereof. Thus, it is difficult to use the shadow mask multiple times, and defects occur during formation of a pattern of an organic light emitting layer. Thus, methods of replacing the shadow mask are required.
As one of the methods of replacing the shadow mask, a tandem white organic light emitting device, hereinafter referred to as white organic light emitting device, has been proposed. The white organic light emitting device will be described.
The white organic light emitting device is characterized in that respective layers between an anode and a cathode are deposited without using a mask during formation of light emitting diodes, i.e., organic layers including an organic light emitting layer are sequentially deposited in a vacuum state by varying components thereof.
Meanwhile, in order to implement the white organic light emitting device, white light may be realized by mixing colors of light emitted from at least two light emitting layers. In this case, the white organic light emitting device includes different light emitting layers emitting a plurality of colors between the anode and the cathode. A charge generation layer is disposed between the light emitting layers, and a stack is distinguished according to each light emitting layer as a unit structure.
In the white organic light emitting device, light is not produced by use of a single material. Instead, the white organic light emitting device emits colored light through combination of colors emitted from a plurality of light emitting layers, which is disposed at different positions in the white organic light emitting device and includes light emitting materials exhibiting photoluminescence (PL) peaks at different wavelengths. A method of implementing a white organic light emitting device including a stack including a fluorescent light emitting layer and a phosphorescent light emitting layer which are sequentially stacked has been reported.